The present invention is related to the field of image processing, particularly medical imaging, and, more specifically, is related to a method and apparatus to achieve direct temporal encoding of spatial information.
The invention has particular application in the field of magnetic resonance imaging (MRI) and will be described in that context. In magnetic resonance imaging, the subject to be imaged is positioned in a strong magnetic field, produced, for example, in the bore of a superconducting electromagnet, and the protons of hydrogen atoms in water and fat tissue and of other magnetic resonant (MR) active nuclei align parallel and anti-parallel to the main magnetic field. These protons precess around the direction of the field at a characteristic angular frequency (the Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the gyromagnetic constant γ of the nucleus).
A transmit coil applies pulses of radio frequency (RF) energy at the Larmor frequency in a direction orthogonal to the main field to excite precessing nuclei to resonance, which results in the net magnetization of all MR active nuclei being flipped from the direction of the main magnetic field into a direction having a transverse component in which it can be detected by the use of a receive coil.
The received signal can be spatially encoded to produce two-dimensional (slice) or three-dimensional (slab) information about the distribution of MR active nuclei and hence of water and tissue.
Typically, a patient is slid axially into the bore of a superconducting magnet, and the main magnetic field is set up along the axis of the bore, termed by convention the Z-direction. Magnetic field gradients are set up, for example, in the Z-direction, to confine the excitation of MR active nuclei to a particular slice in the Z-direction and, in the horizontal X and the vertical Y directions, to encode the resonant MR nuclei in the plane of the slice. An RF transmit coil applies an excitation pulse to excite the protons to resonance, and RF receive coils pick up relaxation signals emitted by the disturbed protons.
To encode/decode received signals in the Y-direction, the signals are detected in the presence of a magnetic field gradient, termed a read-out gradient, to enable different positions of relaxing nuclei to correspond to different precession frequencies of those nuclei about the direction of the main magnetic field due to the influence of the gradient. The data is digitized, and so for each RF excitation pulse, a series of digital data points are collected, and these are mapped into a spatial frequency domain known as K-space. Each RF pulse permits at least one column of digital data points to be collected.
To encode/decode the received signals in the X-direction (read-out dimension), after each RF pulse has been transmitted and before data is collected with the read-out gradient applied, a magnetic field gradient in the Y-direction (phase encoded dimension) is turned on and off. This is done for a series of magnitudes of magnetic field gradients in the Y-direction, one RF pulse typically corresponding to a different magnitude of gradient in the X-direction.
On the generated K-space matrix, the columns of data points correspond to RF pulses followed by different magnitudes of phase-encode gradients.
The field of view imaged by the magnetic resonance imaging apparatus depends on the spacing (in k-space) of the data points in the phase-encode and read-out directions, and the resolution of the image depends on how far the points extend in each direction (in k-space) e.g. how large the maximum phase-encode gradient is.
The signals received from the RF receiver coils are subject to a two dimensional fast Fourier Transform in Fourier Transform processors to produce pixelated images which are stored in image memories. The processing of the signals by Fourier Transform adds a time delay between receiving the signals and viewing an image represented by the signals, thus increasing the time the patient must be subjected to the procedure.
Moreover, to enable the mathematical transformation by the Fourier Transform to accurately represent the desired image, the number of samples taken, the timing of the samples, the total time of signal acquisition, and the magnetic gradient strengths during readout must all be considered and controlled. Variance of these parameters will vary the resolution and field of view of the image.
It would be advantageous if the signals from the RF receiver coils were not subject to a mathematical transformation to produce the images, but could be encoded as images directly, thus eliminating the time delay from the mathematical processing and the need to consider and control these other parameters to enable the mathematical process to produce projection data representative of the image.